Using quantum key distribution can detect radar jamming, at least in theory.

Radar is, broadly speaking, the standard way to recognize and identify incoming objects. Aircraft and ships usually broadcast a signal that identifies them anyway, but even in the absence of that signal, you still want to ensure that you accurately identify passing aircraft—and not by the wreckage they leave after you have shot them down.

This is also critical because the approaching aircraft could broadcast a signature that makes it look innocent when, in fact, it isn't. This form of sophisticated jamming would be very difficult to detect using a standard radar system. When you add the magic of quantum, however, life suddenly becomes a lot harder for the jammer.

The nice thing about an imaging radar system is that you can get the speed, direction, and the shape of the object from different aspects of the signal. The doppler shift on the radar signal gives you speed, the time between sending a pulse and receiving the scattered radiation at your detector gives you distance. The signal intensity from several detectors allows you to create an image. And repeated measurements tell you where the object is going as well as the speed again.

But, since radar is an active signal, it is possible to calculate a pattern of signals to send from the target to make it appear like something it's not. A plane equipped with the right transmitters could spoof its identity via radar. The ideal solution to this would be for the radar transmitter to tag each photon of microwave energy so that the it can verify that the radiation it detects comes from its transmitter. And this is exactly what a group from University of Rochester claims to have done (but hasn't).

What they noted is that a quantum key distribution system offers a way to analyze the statistics of detected photons and determine if they came from the intended source. The key to the security comes from the nature of a quantum measurement. In a classical measurement, you ask questions like "How long is that table?" and get an answer. In quantum mechanics, measurements don't work that way. Instead you ask the question "Is the table 1.60m long?" and the answer is either "yes" or "no."
The same holds for polarization. You place a polarization sensitive mirror in the path of the light. The mirror reflects horizontally polarized photons and transmits vertically polarized photons. This measurement can only report that the photons are vertically or horizontally polarized, even if they have an entirely different polarization, such as 45°. And, no matter what orientation the photon is, the detector will always provide a horizontal or vertical answer.

Now consider this from the perspective of someone who wants to jam a radar system. They want to detect the incoming photons and broadcast new ones so that they show incorrect radar information. But, if they're trying to match the polarization, they have to choose what measurement to make on the incoming photons. And, no matter what choice they make, they will always get an answer, even though it may be entirely wrong. As a consequence, their jamming signal will almost certainly contain enough photons with the wrong polarization to make it easily detectable.

Every photon transmitted by the radar unit is polarized either along the vertical/horizontal direction, or along the two diagonals (diagonal and anti-diagonal). This choice of which orientation is used is randomized for each photon. The jammer has a 50 percent chance of choosing the right polarization orientation. If they choose correctly, the photon that they send back will be indistinguishable from the one sent from the radar system.

Now, let's say the jammer chooses to measure diagonally when the sender is sending horizontally polarized photons. The photon will be detected as either diagonal and anti-diagonal with 50 percent probability. The jammer duly resends whichever polarization the detector told it to, with incorrect image information.

Meanwhile, the detector on the radar unit knows that the photon should be horizontal and is set up accordingly. The jammer has sent a photon that is actually in one of the diagonal states. Once again, even though it's diagonally polarized, the photon will be detected as either vertical or horizontal. Half the time, the photon is measured to be vertically oriented, which reveals the presence of the jammer.

The upshot is that 25 percent of the time, the radar unit receives a photon with the wrong polarization. (The jammer makes the wrong choice half the time, and the photon goes the wrong direction in the receiver's detector half the time—combined, they account for the 25 percent.) Such a high error rate is clearly observable compared to imaging without the jammer.

And, it works, of course. The researchers showed that they could tell the difference between jammed images and unjammed images very easily. And polarization is an ideal choice because it is not used for other aspects of radar, so the verification scheme does not get in the way of the radar measurements.

But we're apparently not ready for a jam-proof radar. The team did this with lasers rather than an actual radar system. Why? Well, they wanted a proof-of-principle* demonstration and they are an optics group. There is also the limitation that, although you know you are being jammed, you still end up with the image that the jammer projects. So, you still have no idea what is out there or where, but at least you know not to trust your radar image.

I am very skeptical that this will ever see the light of day outside of the lab. You would need a single photon source, and even with efficient single photon sources and detectors, there would be problems. On reflection from a 3D object, the polarization of the photon will be altered. What's more, that change will be different for every photon, since the object will be moving and changing orientation.

It is certainly possible that the changes are small enough that the error rate doesn't reach the 25 percent threshold—but if these reflections increase the error rate to 25 percent, then the real object would be indistinguishable from the jammer's projected object.

Still, the entire world of science is devoted to proving curmudgeons like me wrong and I look forward to that happening again.

*Proof-of-principle is science code for "we don't have a clue how to do this in any useful context, so we will do something useless and pretend that the rest is easy."

47 Reader Comments

Off topic but maybe and interesting side story: Those EA-6Bs (article pic), and their cousin the A6 had to have been the loudest aircraft I've ever heard while taking off from an aircraft carrier. Quite literally I remember standing just off the wing of those guys when they ramped up to full military power just prior to being launched and can remember every bone in my head rattling. You could feel your teeth rattle in their sockets and almost feel the fused plates in your skull vibrate. All other aircraft, like the F-14s and F-18s, seemed to pale in comparison. Good times.

Why won't it work with pulses of photons instead of individual photons? Isn't the logic the same? Or is it because with multiple photons of the same orientation you could design a detector that would allow the jammer to figure out the orientation of the pulse?

More importantly why are we using Photons instead of radiowaves? Short of we ran out of ideas on what to do with this grant money, and hey, the lasers were sitting right over there.

Most functional Jamming is pretty obvious to the Jammee. They just punch down a whole lot of energy at the sets and hope to fuzz them out/give them so many false contacts they can't effectively tell whats really there, and what isn't.

Jamming isn't really designed to stop an enemy from knowing you're coming to cook his bacon. Its to keep your bacon from being cooked enroute. (And occasionally screw with him when indeed you are not planning to cook, yet)

Which doesn't make this less cool. Just exceptionally questionable in research fund useage.

Major problem: they did it with optical wavelengths because you can detect single photons of those wavelengths. But radar uses much longer waves (by a factor of a million), meaning that the photons have a million times less energy, and as far as I know it's not possible to detect single photons of 1-meter EM radiation and measure their polarization individually.

Actually LIDAR is a widely used remote sensing technique that uses lasers instead of microwaves and radio waves. Besides, all those quantum computing papers that use Shor's algorithm to factorize, say, 15 are "just" proof-of-concept experiments too. Research is incremental, lighten up.

More importantly why are we using Photons instead of radiowaves? Short of we ran out of ideas on what to do with this grant money, and hey, the lasers were sitting right over there.

Most functional Jamming is pretty obvious to the Jammee. They just punch down a whole lot of energy at the sets and hope to fuzz them out/give them so many false contacts they can't effectively tell whats really there, and what isn't.

Jamming isn't really designed to stop an enemy from knowing you're coming to cook his bacon. Its to keep your bacon from being cooked enroute. (And occasionally screw with him when indeed you are not planning to cook, yet)

Which doesn't make this less cool. Just exceptionally questionable in research fund useage.

There are many different technologies and techniques bundled under this ubiquitous term "radar jamming". Some techniques are designed to hide the identity and indeed the very location of the jammer aircraft. No of course it isn't stealth. The other side would know something is up, but would not know what or where, and that can be just as useful as stealth. On the most basic level the jammers can prevent radar lock-ons until it's too late. But they can also do so much more. Oh man. I need to bite my tongue here. ... this stuff is terribly cool.

At first I thought using polarized light was dumb. After all, reflections off metal aren't polarized. Any photographer worth their salt could tell you that. On further investigation it looks like metallic reflection just preserves the prevalent unpolarized state of light.

My question now is: does this mean the technique won't work at all on future composite aircraft?

It's incredibly important to know if you're being jammed, so that knowledge alone is very valuable.

What if the enemy is not interfering with your normal radar operations, but instead projecting a different image than what's actually there? Additional, false contacts? So, this does seem fairly valuable research, if a few steps removed from implementation.

Actually LIDAR is a widely used remote sensing technique that uses lasers instead of microwaves and radio waves. Besides, all those quantum computing papers that use Shor's algorithm to factorize, say, 15 are "just" proof-of-concept experiments too. Research is incremental, lighten up.

LIDAR, which we use where we work, is simply RADAR with a shorter wavelength RF carrier frequency that happens to be coherent by it's nature. What is being discussed is using the quantum nature of, or otherwise a deeper understanding of, quantum properties to further enhance the separation of signal from noise.

More importantly why are we using Photons instead of radiowaves? Short of we ran out of ideas on what to do with this grant money, and hey, the lasers were sitting right over there.

Most functional Jamming is pretty obvious to the Jammee. They just punch down a whole lot of energy at the sets and hope to fuzz them out/give them so many false contacts they can't effectively tell whats really there, and what isn't.

Jamming isn't really designed to stop an enemy from knowing you're coming to cook his bacon. Its to keep your bacon from being cooked enroute. (And occasionally screw with him when indeed you are not planning to cook, yet)

Which doesn't make this less cool. Just exceptionally questionable in research fund useage.

You're right that full-on jamming is obvious, but there's various spoof attacks against radar that are hard to distinguish from actual returns.

Wow. F18s fly above my workplace with some regularity and those suckers are LOUD. Somewhat hard to imagine louder.

that sounds like it would be cool, for a day.

No, it's cool every day. I used to work at an airport with a National Guard base attached. Loved to hear the F16s take off every afternoon for training. (sucked when I got moved away from my window and I couldn't watch them any more)

[A6s are loud] All other aircraft, like the F-14s and F-18s, seemed to pale in comparison. Good times.

Wow. F18s fly above my workplace with some regularity and those suckers are LOUD. Somewhat hard to imagine louder.

Louder? I give you the Bone (B1 bomber) buzzing the airfield at full mil power. OMFG. That was loud.

I have been right under a 4-ship of F-18s. Not so loud.

The loudest aircraft ever made is a Soviet (not surprising because they liked to out-do the US by seconds and inches and fractions of decibel just so they could claim they did it) Tupolev Tu-95 "Bear" bomber. That fucker has 8 counter-rotating SUPERSONIC propellers. Yeah the props are spinning at supersonic speeds.

This radar only guarantees integrity of the packets, and integrity is very hard to do against a non-cooperative target. As an analogy, I would say this is similar to trying to connect to a secure site against a person who is trying to do a MITM attack against you. The bad guy alters the packets so you keep getting an “invalid certificate”. The site's integrity works just fine, but you now can't use the site unless you ignore the integrity. If I were to try to jam against it, I would create a jammer that sends out interference in the same frequencies of the radar. This would (hopefully) alter the radar's signal bouncing off of me enough such that it would reject everything. Then the radar will not see anything.

This radar also only addresses false target jamming. It has no way to stop noise jamming, and noise comes from all parts of the system and is purely random (background radiation, thermal noise, etc). Due to this, noise jamming continues to work on it.

Why won't it work with pulses of photons instead of individual photons? Isn't the logic the same? Or is it because with multiple photons of the same orientation you could design a detector that would allow the jammer to figure out the orientation of the pulse?

Hmm, isn't it almost impossible to jam modern AESA radars like those found in the F22 and F35 anyway, at least without a massively powerful transmitter sending broadband noise?

That is what I thought as well, but I am no where near an expert on this.

From what I understand AESA radars transmit over a huge range of frequencies, and can randomly change frequencies every pulse. Since the target can't predict which frequencies the radar will be transmitting, it would have to jam all possible frequencies.

So what about this: If the detector is sending horizontally or vertically polarized photons, and the jammer has two detectors on it - a vertical/horizontal and a diagonal - cannot the jammer can see both which detector is getting a 50% error rate, and which one a 100% correct rate, and then respond with the appropriate polarization?

The only way I could see this not working is if the detector was sending both vertical and horizontal at the same time (50% of each, randomized in a way the detector could interpret, which I assume is impossible, due to them being received out of order); this way both of the jammer's detectors would see a 50% error rate.

Unless you can avoid the out of order return situation, it seems pretty trivial - not impossible - to spoof radar systems in this manner with a pair of vertical/horizontal and diagonal/antidiagonal detectors on the jammer.

[A6s are loud] All other aircraft, like the F-14s and F-18s, seemed to pale in comparison. Good times.

Wow. F18s fly above my workplace with some regularity and those suckers are LOUD. Somewhat hard to imagine louder.

Louder? I give you the Bone (B1 bomber) buzzing the airfield at full mil power. OMFG. That was loud.

I have been right under a 4-ship of F-18s. Not so loud.

The loudest aircraft ever made is a Soviet (not surprising because they liked to out-do the US by seconds and inches and fractions of decibel just so they could claim they did it) Tupolev Tu-95 "Bear" bomber. That fucker has 8 counter-rotating SUPERSONIC propellers. Yeah the props are spinning at supersonic speeds.

Sorry for the off-topic post, but I had to contribute to this...I have been under a B1 doing flyby's when KI Sawyer AFB still existed...knocked stuff off the mantle in my house. That is still the loudest thing I have ever heard, short of a sonic boom.

Also, when KI would scramble a squadron of B-52s and KC-135s on drills...that was an epic drone, too, though more due to how long it lasted than overall volume.

I was wondering, wouldn't the expected value of the percentage of the wrongly polarized photons only be something like 12.5%. I mean the max error percentage of 25% would be when the polarisers on the detector and receiver were 45 degrees out of phase decreasing to zero percent miss-sent photons when they where perfectly aligned. with a uniform distribution of phase difference that would work out to 12.5% average error rate would it not? someone please correct me if I'm wrong

Hate to burst yalls bubble, which is actually hard without a halfass answer starting with "according to the journal of electronic defense on X date..." This has been done over and over. The same problems remain. Lower tech higher power solutions still defeat the new fancy way of creating a unique transmission. When you are looking for a microwatt return in the grass and someone comes along with a computer controlled blast cannon you have a real hard time getting the tiny from the big at all. I'm sure it works great transmitting in the clear, but so does a signal without any changes.

And then finding enough data to pick off the digital signature is even harder IF you can detect the bounce. Until you can stare at the sun from earth and identify your flashlight next to a billion other ones with a pair of glasses, I'm not buying it. Just as your eyes would be destroyed in the process, you fry a lot of hardware trying to get this stuff to work. The filtering and detection gear will not ever catch up to the transmission gear. It is a new toy to sell for too much money with the same end result as the last 300. But I guess it provides jobs so that is good right?

Chris Lee / Chris writes for Ars Technica's science section. A physicist by day and science writer by night, he specializes in quantum physics and optics. He lives and works in Eindhoven, the Netherlands.